Filler for use in paper manufacture and procedure for producing a filler

ABSTRACT

The invention relates to a filler used in paper manufacture and mainly consisting of calcium carbonate, and to a procedure for producing the filler. The filler consists of porous aggregates formed by precipitated calcium carbonate particles. In the procedure, calcium carbonate is precipitated.

This application is a continuation of U.S. application Ser. No.09/726,843, filed on Nov. 30, 2000 now U.S. Pat. No. 6,375,794. U.S.application Ser. No. 09/726,843 is a continuation of U.S. applicationSer. No. 08/973,926, filed on Mar. 5, 1998, and issued as U.S. Pat. No.6,251,222 on Jun. 26, 2001. The entire disclosure of U.S. applicationSer. No. 09/726,843 and U.S. application Ser. No. 08/973,926 areincorporated herein by reference.

The present invention relates to a filler for use in paper manufactureas defined in the preamble of claim 1. Moreover, the invention relatesto a procedure for producing said filler.

In the present application, ‘paper’ refers to various kinds of paper andcardboard, manufactured with paper and cardboard machines, coated oruncoated.

Today, the direction of development of paper products is to anincreasing degree determined by customers and legislative measures. Thebuyers of printing paper want to reduce the postage expenses and theamount of waste produced. Further, packages are subject to wasteprocessing charges dependent on weight. Generally, it seems that energytaxes and environmental protection taxes are being imposed on the priceof paper products as a permanent extra encumbrance. For these reasons,paper buyers want products which have a lower grammage while stillmeeting high quality standards.

Specification FI 931584 presents a composite product based on chemicalpulp fibre or mechanical pulp fibre, with calcium carbonate crystalsprecipitated onto the surface of the product. Further, specification FI944355 presents a precipitated calcium carbonate, which is in the formof calcite particle aggregates where at least 25% of the particles areof a prismatic shape. Precipitation is performed using a seed material.Further, specification EP 0604095 presents a procedure for theprocessing of waste material whereby calcium carbonate is precipitatedonto the surface of waste material containing inorganic matter; thewaste material may contain organic waste fibre, e.g. waste fibrecontained in the effluent of a paper mill, with fibre length below 75μm. The calcium carbonate products described in the specificationsreferred to are intended to be used as fillers in paper manufacture.

In the manufacture of high-quality paper, the aim is to produce thepaper with a minimum amount of raw material. When the grammage of thepaper is reduced, its opacity becomes a critical factor. The opacity canbe increased by increasing the filler content of the paper, which,however, generally reduces its strength. Therefore, the aim is to alterthe structure of the paper while at the same time preserving theimportant good product qualities. For paper based communication toremain competitive in relation to electric communication, the printingquality of paper products has to be further improved.—These generaldevelopment trends impose very high requirements on the raw materialsand manufacturing processes used in paper production. To meet therequirements, very intensive efforts have been made in recent times todevelop paper raw materials and manufacturing processes.

The object of the present invention is to produce a new kind of calciumcarbonate based filler for paper manufacture that meets the criteriadescribed above.

A further object of the invention is to produce a new kind of calciumcarbonate based filler which has better optic properties than earliercalcium carbonate based fillers.

A further object of the invention is to produce a new kind of calciumcarbonate based filler which gives the paper better strength properties,especially a better tensile strength, than earlier calcium carbonatebased fillers.

A further object of the invention is to produce a new kind of calciumcarbonate based filler which gives the paper a lower grammage thanearlier calcium carbonate based fillers.

A further object of the invention is to produce a new kind of calciumcarbonate based filler which has a higher retention than earlier calciumcarbonate based fillers.

A further object of the invention is to produce a new kind of calciumcarbonate based filler which reduces the overall costs of papermanufacture.

An additional object of the invention is to present a procedure for themanufacture of said filler.

As for the features characteristic of the invention, reference is madeto the claims.

The invention is based, among other things, on the fact, established viacorresponding investigations, that calcium carbonate can be precipitatedin a way that causes it to effectively adhere to fibres and noilfibrils. The precipitation can be so performed that porous calciumcarbonate aggregates held together by fibrils, i.e. fine fibres, areformed, which aggregates contain plenty of empty space and in which thecalcium carbonate particles have precipitated onto the noil fibrils,adhering to them. The noil fibrils with calcium carbonate particlesprecipitated on them form fibres resembling pearl necklaces, and thecalcium carbonate aggregates resemble clusters of pearl necklaces. Theaggregates have a very large ratio of effective volume to mass ascompared with the corresponding ratio of conventional calcium carbonateused as filler; effective volume here means the volume taken up bypigment in the paper.

The noil fibrils used in the filler of the invention are obtained fromcellulose fibre and/or mechanical pulp fibre. The fibrils are producedfrom cellulose fibre and/or mechanical pulp fibre by refining.Furthermore, the noil fibrils are preferably divided into fractions,thickness 0.1-2 μm, length mainly 10-400 μm, suitably 10-300 μm,preferably 10-150 μm. Thus, the noil fibrils consist of cellulose fibreand/or mechanical pulp fibre, which means that they contain nosignificant amounts of inorganic matter, preferably no inorganic matterat all.

The diameter of the calcium carbonate particles in the aggregate is ofthe order of about 0.2-3 μm, preferably about 0.3-1.5 μm.

The diameter of the CaCO₃ crystal aggregates is of the order of about2-10 μm.

Cellulose based noil also contains roundish noil particles which, afterthe precipitation process, are covered with calcium carbonate particles.In this case, as to its properties, a particle of calcium carbonatefiller corresponds in the first place to a hollow filler particle havinga small unit weight. In reality, the pigment is not completely hollow,because it contains noil; however, the noil has a lower unit weight thancalcium carbonate, therefore the particle has a very low unit weight.

The new precipitated, calcium carbonate based filler of the inventionbestows paper better optic properties and a clearly greater strengththan prior-art calcium carbonate based fillers do. Furthermore, thefiller of the invention allows the filler content of paper to beincreased without impairing its other properties, e.g. theaforementioned strength properties, such as tensile strength. This is asignificant contribution towards lowering the grammage of paper.

Further, the new filler of the invention has a clearly better retentionin paper manufacture than prior-art calcium carbonate based fillers.

In consequence of the aforesaid factors, it is generally possible toachieve cost savings in paper manufacture by using the filler of theinvention.

In prior art, light filler pigments are known, e.g. hollow plasticpigments, which are supposed to provide the same advantages as thecalcium carbonate based filler of the present invention. However,plastic pigments are expensive, which restricts their use. When thefiller of the invention is compared with pore filled or lumen filledfibre, it is to be noted that, unlike in the case of aforesaid fibres,the calcium carbonate in the filler of the invention is not-insideindividual noil fibres but on the surface of the noil. In addition, themass ratio of calcium carbonate and fibrous matter is much larger in thefiller of the invention than in pore filled or lumen filled fibre. Thus,the filler of the invention is a completely new product, and it shouldnot be confused with prior-art pore filled or lumen filled fibre.

The filler of the invention and the procedure for its manufacture differfrom the specification FI 931584 referred to in the introductionespecially on the basis of the thickness and length of the noil fibrils,i.e. in the present application, the noil fibrils have been refined witha pulp refiner. From the filler and manufacturing method described inspecification FI 944355, the filler and manufacturing method of thepresent invention likewise differ in respect of the refined noilfibrils; in addition, in the specification referred to, the calciteparticles are of a prismatic shape and their production requires the useof a special seed material. With respect to the product and procedurepresented in specification EP 0604095, the product and procedure of thepresent invention likewise differ on the basis of the refinement andsize of the noil fibrils; in addition, the fibre material used in thespecification referred to is waste fibre containing inorganic or othermatter.—In general, in addition to the differences stated above, thefiller of the present invention differs from those described in thereference specifications on the basis of the good optic properties,strength properties and very high retention achieved, and especially onthe basis of the combination of exceptionally good optic and strengthproperties.

The fibre used in the procedure of the invention may consist ofchemical, mechanical or semi-mechancial pulp produced by any pulp orpaper manufacturing method known in itself, or a combination of these inarbitrary proportions, the proportion of each component being 0-100% byweight. The refining of the pulp into noil fibrils can be implementedusing any pulp refiner known in itself in pulp processing industry. Whendesirable, the refined noil fibrils can be screened by any fractionatingmethod known in itself in pulp processing, e.g. using a wire screen,into the desired fibril size.

In precipitation, the noil, i.e. e.g. pulp based or other fibre basednoil, is refined with a pulp refiner and screened, preferred fractionsbeing e.g. wire screen fractions P100-P400.

In the procedure of the invention, calcium carbonate can be precipitatedfrom any suitable solution or mixture, e.g. from a mixture of Ca(OH)₂water solution and solid Ca(OH)₂ or from a calcium hydroxide watersolution. Thus, precipitation can be implemented using any substancethat precipitates calcium carbonate, e.g. carbon dioxide, such asgaseous carbon dioxide, suitably 1-100%, preferably 10-100% carbondioxide gas. Instead of calcium hydroxide and carbon dioxide, it ispossible to use any reaction producing calcium carbonate, e.g. thereaction between calcium chloride and sodium carbonate, producingcalcium carbonate and sodium chloride.

The precipitation of calcium carbonate is performed on the surface ofnoil originating from cellulose fibre, suitably noil fibrils. Theconcentration of noil in the precipitation process is suitably 0.0001-18w-%, preferably 0.4-10 w-%. When calcium hydroxide is used, the massratio of calcium hydroxide and cellulose fibres in precipitation issuitably 0.1-20, preferably 1.4-4. The precipitation temperature is inthe range 5-150° C., suitably 10-90° C., preferably 15-80° C.

In the carbon dioxide method, the net reaction is

Ca(OH)₂+CO₂⇄CaCO₃+H₂O.

In the chloride method, the net reaction is

CaCl₂+Na₂CO₃⇄CaCO₃+2NaCl

Calcium carbonate precipitates when the calcium compounds reactaccording to the reaction equations. It is possible to influence thecrystal size and/or shape by adjusting the reaction conditions.

The precipitation can be advantageously effected in a specific reactorwhere e.g. calcium hydroxide and noil are mixed. The carbonationreaction is implemented by supplying carbon dioxide, e.g. gaseous carbondioxide, into the reactor. The progress of the reaction can be monitoredby measuring the pH and conductivity of the mixture. The mixing and thesupply of gas can be terminated when the pH of the mixture has fallen tothe value of about 7.5, depending on the pH value of the noil. Thecarbonation is carried out e.g. in a water solution or mixture ofCa(OH)₂.

If desirable, it is possible to add a dispersing agent, e.g. sodiumhexametaphosphate (Na-HMF) or other dispersing agent(s), into the fillerproduced.

The filler of the invention can be used as filler as such or in anyratio of mixture (0-100%) with another filler or other fillers. Theamount of filler used in paper is 0.1-50 w-%, preferably 0.1-30 w-%.

The filler of the invention and the procedure for its manufacture aredescribed in more detail in the following embodiment examples byreferring to the attached drawings, in which

FIG. 1 presents the equipment used in the procedure of the invention;

FIGS. 2-4 present pictures taken of the filler of the invention by meansof an electron microscope;

FIGS. 5-8 present graphs representing the properties of the filler ofthe invention as compared with those of a prior-art filler.

EXAMPLE 1 Production of Filler

Bleached pine sulphate pulp was refined in a Valley laboratory hollanderin accordance with the SCAN-C 25:-76 standard for 2.5 hours. The refinedpulp was screened by means of a Bauer-McNett screen, initially using thewire sequence 14-50-100-200 mesh. The amount of dry matter screened at atime was 45 g. The fraction passed through the 200 mesh wire (P200fraction) was saved and allowed to settle for 2 days, whereupon theaqueous phase on its- surface was separated.

The P200 fraction was further fractionated with the wire sequence100-200-290-400 mesh. The 100 mesh wire was used to equalize thescreening process and prevent the 200 mesh wire from getting blocked atthe initial stage. The fraction passed through the 400 mesh wire (P400fraction) was saved and, after the noil fraction had settled, theaqueous phase on the surface was separated.

The P400 fraction was thickened by centrifugation to a consistency of4.7 g/l, whereupon the noil was ready for use in the production of afiller.

The filler was produced in a mixing tank reactor 1, FIG. 1. The reactorhad a capacity of 5 liters and its temperature could be regulated via awater circulation system 2 in its casing. The reactor contained fourvertical foul plates designed to increase the mixing efficiency. A gasmixture of carbon dioxide and nitrogen was supplied via a pipe 3 to apoint below the mixer element. The flow and carbon dioxide content ofthe gas mixture could be adjusted by means of control valves 5, 6provided in the gas pipes 3, 4. Measuring sensors 7, 8, 9 were placed inthe reactor via holes in the cover. The measuring elements wereconnected to a computer 10, in which the measurement data was collectedand stored.

Precipitation was performed at a temperature of 35° C. and the carboncontent of the gas mixture was adjusted to a value of 15% by volume; thereaction volume was 3.2 l.

The filler was produced using three different Ca(OH)₂/noil ratios. Theproportions of raw materials are presented in Table 1.

TABLE 1 Proportions of raw materials Precipi- Precipi- Precipi- tation 1tation 2 tation 3 m_(noil), g 15 15 15 m_(Ca(OH)2), g 22 35 50V_(nitrogen), l/min 5.25 8.36 11.94 V_(carbon dioxide), 0.93 1.48 2.11l/min

Before the reaction was started, the noil was homogenized by mixing itin the reactor for 5 min at a mixing speed of 600 l/min. At this stage,a small nitrogen flow was used to prevent the gas pipes from gettingblocked. After this, the mixing speed was adjusted to the value 1000l/min, and calcium hydroxide was added into the reactor. The measuringsensors were placed in the reactor and the reaction was started byopening the CO₂ flow as well. The progress of the reaction was monitoredby measuring the pH, 8, and conductivity, 9, of the mixture. The mixingand gas supply were stopped when the pH, 8, had fallen to the value 7.5.

Pictures of the product obtained were taken with an electron microscope(SEM), FIGS. 2, 3, 4. From the SEM pictures it can be seen that theproduct consists of porous calcium carbonate aggregates held together bynoil fibrils and containing plenty of empty space, with CaCO₃ particlesprecipitated onto the noil fibrils, adhering to them. The calciumcarbonate particles in the aggregate have a diameter of 0.3-1.5 μm and aroundish and partly shuttle-like shape. The diameter of the aggregatesvaries between about 2-10 μm. The noil/CaCO₃ fibrils can be described asresembling pearl necklaces and the aggregates as resembling clusters ofpearl necklaces. There are also roundish noil particles (FIG. 3), andthese are covered with tiny CaCO₃ particles. In this case, we can evenspeak of a hollow CaCO₃ pigment, which has a low unit weight (thepigment is not completely hollow because there is some noil inside it;however, noil has a lower unit weight than calcium carbonate). Accordingto an X-ray diffraction analysis, 100% of the precipitated calciumcarbonate consisted of calcite.

EXAMPLE 2 Properties of Paper

To test the technical potential of the filler in paper manufacture, aseries of sheet tests were performed, in which the properties of thepaper were compared when two fillers as provided by the invention andcalcium carbonate fillers already available in the market, PCC (AlbacarLO) and GC (Fincarb 6005) were used.

For the production of laboratory sheets, a pulp mixture containing 75w-% bleached mechanical pulp and 25 w-% bleached pine sulphate pulp wasprepared. The pulp was refined in a Valley laboratory hollander to SRnumber 30 in accordance with the SCAN-C 25:76 standard, the refiningtime being 38 min.

The fillers obtained from precipitations 1 and 2 presented in FIG. 1were used undiluted in the production of laboratory sheets, and thefiller from precipitation 3 was diluted to one half of its consistencyafter precipitation. For the production of reference sheets, solutionshaving a consistency of 25 g/l were prepared from commercial CaCO₃fillers.

In a laboratory sheet mould, 60 g/m² sheets were produced withoutcirculation water according to the standards SCAN-C 26:76 and SCAN-M5:76 except for drum drying and corresponding wet pressing of thesheets. As retention agents, cationic starch (Raisamyl 135) 0.65% andsilica 0.15% of the fibre mass were used.

For wet pressing corresponding to drum drying, the sheets were piled upas follows:

Top of pile→press plate 2 dried blotting boards new blotting boardlaboratory sheet couching board 2 dried blotting boards

Bottom of pile→press plate

The sheet pile was placed in the press and it was pressed by applying apressure of 490±kPa to the sheets for 4 minutes. After the wet pressing,the blotting boards on either side of the sheets were left sticking tothe sheets and the sheets were placed in a cold drying drum. The sheetswere dried in the drum at a temperature of 1000° C. for 2 h. After thedrying, the blotting boards were released from the sheets and the sheetswere seasoned for at least 24 h at a temperature of 23±1° C., therelative humidity being 50±2%.

For the finished sheets, the calcium carbonate content, grammage, ISOwhiteness, light-scattering coefficient and tensile index weredetermined. The results are presented in Tables 2, 3 and 4.

TABLE 2 Paper properties obtained using a filler as provided by theinvention Precipi- Precipi- Precipi- tation 1 tation 2 tation 3 CaCO₃content, % 10.2 15.8 12.1 17.1 12.8 17.7 grammage, g/m² 65.0 66.3 65.165.6 65.0 66.0 ISO whiteness, % 72.4 75.0 73.1 75.9 74.0 76.8light-scattering 74.4 82.6 77.0 87.2 78.6 90.3 coeff., m²/kg tensileindex, 48.8 47.4 50.0 44.9 45.4 40.7 Nm/g

TABLE 3 Paper properties obtained using commercial CaCO₃ fillers PCC GCCaCO₃ content, % 11.8 18.3 22.9 11.6 18.0 22.0 grammage, g/m² 65.1 68.366.7 67.5 63.6 68.4 ISO whiteness, % 73.1 75.0 76.1 72.6 73.7 74.5light-scattering 76.8 85.8 88.4 72.8 77.5 82.4 coeff., m²/kg tensileindex, 33.3 28.4 26.6 36.9 31.6 28.7 Nm/g

Calcium carbonate retention was on an average 92% for the filler of theinvention, 64% for commercial precipitated calcium carbonate (PCC) and62% for commercial ground calcium carbonate (GC).

TABLE 4 Paper properties with mere pulp without filler CaCO₃ content, % 0   grammage, g/m² 64.7 ISO whiteness, % 71.2 light-scattering coeff.,m²/kg 62.0 tensile index, Nm/g 56.2

FIGS. 5-8 present the results in the form of graphs. In FIGS. 5-8, thereferences S1, S2 and S2 correspond to the results shown in the tablewith fillers obtained from precipitations 1, 2 and 3, respectively; thereferences PCC and GC indicate results obtained with commercialprecipitated calcium carbonate and ground calcium carbonate,respectively. FIGS. 5 and 6 indicate that the filler of the inventionhas better optic properties as compared with the correspondingproperties with commercial CaCO₃ fillers used in the same CaCO₃contents. FIG. 7 shows that the tensile strength in the case of thefiller of the invention is clearly better than in the case of commercialCaCO₃ fillers used in the same CaCO₃ contents. In addition, FIG. 8 showsthe light-scattering coefficient as a function of the tensile index. Inthis survey, both the optic properties of the paper and its runnabilityon a paper machine are considered. In this survey, the filler of theinvention is clearly better than commercial CaCO₃ fillers. In otherwords, with the same light-scattering coefficient values, the filler ofthe invention gives a clearly better tensile strength than do commercialCaCO₃ fillers. From the graphs we can see a trend of improving opticalproperties and decreasing tensile strength of the paper as the ratiom_(CA(OH)2)/m_(noil) in precipitation increases.

The excellent properties of the new type of porous CaCO₃ fillerdescribed in the foregoing allow the CaCO₃ content to be increased andthe grammage of paper to be further reduced without impairing otherimportant qualities of paper. Considering the improved retention of thefiller of the invention in paper manufacture, the aforesaid good resultstogether also allow cost savings to be achieved.

The embodiment examples are intended to illustrate the invention withoutlimiting it in any way.

What is claimed is:
 1. A method for producing filler for use in papermanufacture, the method comprising steps of: (a) refining cellulosefibre, mechanical pulp fibre, or mixture of mechanical pulp fibre intofibrils, wherein: (i) the fibrils having a thickness of between 0.1 μmand 2 μm, and a length of between 10 μm and 400 μm; and (ii) the fibrilsbeing detached from fibers; and (b) chemically precipitating calciumcarbonate onto the surface of the fibrils to provide a porous aggregateof calcium carbonate precipitated onto the surface of fibrils, wherein:(i) the calcium carbonate on the fibrils have a diameter of between 0.2μm and 3 μm.
 2. A method according to claim 1, wherein the step ofprecipitating calcium carbonate onto the surface of the fractionatedfibrils fraction comprises reacting calcium hydroxide and carbondioxide.
 3. A method according to claim 2, wherein the mass ratio ofcalcium hydroxide and fibrils is 0.1:1 to 20:1.
 4. A method according toclaim 2, wherein the mass ratio of calcium hydroxide and fibrils is1.4:1 to 4:1.
 5. A method according to claim 1, wherein the step ofprecipitating calcium carbonate onto the surface of the fractionatedfibrils fraction comprises reacting calcium chloride and sodiumcarbonate.
 6. A method according to claim 5, wherein the mass ratio ofcalcium chloride to fibrils is between 0.15:1 and 30:1.
 7. A methodaccording to claim 5, wherein the mass ratio of calcium chloride tofibrils is between 2.1:1 and 6:1.
 8. A method according to claim 1,wherein said step of precipitating calcium carbonate onto the surface ofthe fractionated fibrils comprises providing a concentration of fibrilsof 0.001 wt. % and 18 wt. %.
 9. A method according to claim 1, whereinsaid step of precipitating calcium carbonate onto the surface of thefractionated fibrils comprises providing a concentration of fibrils of0.4 wt. % to 10 wt. %.
 10. A method according to claim 1, wherein thefractionated fibrils fraction has an average length of between 10 μm and150 μm.
 11. A method according to claim 1, further comprising a step ofscreening the fibrils through a 200 mesh wire.
 12. A method according toclaim 1, wherein the step of precipitating comprising precipitating at atemperature of between 5° C. and 150° C.
 13. A method according to claim1, wherein the step of precipitating comprising precipitating at atemperature of between 10° C. and 90° C.
 14. A method according to claim1, wherein the step of precipitating comprising precipitating at atemperature of between 15° C. and 80° C.
 15. A method according to claim1, wherein the step of precipitating calcium carbonate onto the surfaceof the fractionated fibrils comprises providing a concentration offibrils of 0.0001 wt. % and 18 wt. %.
 16. A method according to claim 1,wherein said step of precipitating calcium carbonate onto the surface ofthe fractionated fibrils comprises providing a concentration of fibrilsof 0.4 wt. % to 10 wt. %.
 17. Porous aggregate prepared according to themethod of claim 1.